Diffractive optical element and method for manufacturing same

ABSTRACT

An element includes a first resin layer and a second resin layer disposed between a first glass lens substrate and a second glass lens substrate, a boundary surface between the first resin layer and the second resin layer having a diffraction grating shape including a plurality of inclined surfaces and wall surfaces. The second resin layer is composed of a fluororesin in which fine metal oxide particles are dispersed. Since a refractive index distribution easily occurs in this material during curing by application of ultraviolet light, by applying ultraviolet light substantially perpendicular to the inclined surfaces of the diffraction grating shape, a refractive index distribution is formed in the thickness direction perpendicularly to the inclined surfaces.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. application Ser. No.12/631,657, filed on Dec. 4, 2009, which claims the benefit of JapanesePatent Application No. 2008-314210 filed Dec. 10, 2008 and No.2009-239394 filed Oct. 16, 2009, which are hereby incorporated byreference herein in their entireties.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a diffractive optical element used foran optical system or the like and a method for manufacturing thediffractive optical element.

2. Description of the Related Art

As one of the methods for correcting chromatic aberration in an opticalsystem, a method has been known in which two lenses composed of glassmaterials that differ in dispersion properties are combined. Meanwhile,another method has been known in which a diffractive optical elementhaving a diffraction effect is provided on a lens surface to therebyreduce chromatic aberration. This method uses a physical phenomenonwhere the refractive and diffractive surfaces in an optical systemexhibit chromatic aberration outputs in opposite directions with respectto a light ray having a given reference wavelength.

Furthermore, in order to adjust refractive indices and Abbe numbers ofdiffractive optical elements, U.S. Pat. No. 6,759,471 (PatentDocument 1) discloses a composite material in which fine particlescomposed of a transparent conductive metal oxide, such as ITO, ATO,SnO₂, or ZnO, are mixed/dispersed in a UV curable binder resin.Furthermore, Patent Document 1 also discloses a laminated diffractiveoptical element formed by laminating two resin layers. In an opticalsystem having a chromatic aberration correction effect, such a laminateddiffractive optical element can greatly reduce diffraction efficiency inthe vicinity of a designed order in the wavelength region to be used.

In recent years, when a diffractive optical element is used as a cameralens, nano-level shape accuracy of a diffraction grating may berequired. However, in the case where a photo-curable resin is used,since the resin starts to react from the points irradiated withultraviolet light or the like, a difference in density occurs in thecured resin due to the difference in the curing rate, resulting in anon-uniform refractive index distribution.

SUMMARY OF THE INVENTION

According to the present invention, there is provided an element whichincludes a glass lens substrate and a resin layer composed of aphoto-curable resin disposed on the glass lens substrate, the resinlayer having a diffraction grating shape including a plurality ofinclined surfaces and wall surfaces. The refractive index of the resinlayer varies depending on a thickness of the resin layer, and thevariation of the refractive index is based on light that appliessubstantially perpendicular to the inclined surfaces.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1D are each a cross-sectional view of a diffraction gratingfor illustrating the principle of the present invention.

FIGS. 2A to 2C are each a cross-sectional view of a diffraction gratingfor illustrating the principle of the present invention.

FIG. 3 is a cross-sectional view of a diffractive optical elementaccording to the present invention.

FIGS. 4A to 4E are cross-sectional views showing a method for forming adiffractive optical element according to the present invention.

FIGS. 5A to 5D are cross-sectional views showing a method for forming adiffractive optical element according to the present invention.

FIGS. 6A and 6B are cross-sectional views showing a method of applyinglight to a diffractive optical element according to a first embodiment.

FIGS. 7A and 7B are cross-sectional views showing a method of applyinglight to a diffractive optical element according to a second embodiment.

DESCRIPTION OF THE EMBODIMENTS

First, the basic principle in carrying out the present invention will bedescribed. As a result of diligent study, the present inventor has foundthat, when a photo-curable resin is cured, due to the difference in thecuring rate, a non-uniform refractive index distribution occurs in theresin. This results from the fact that due to flow of the uncured resinduring curing, a difference in the density of the resin occurs.Furthermore, the non-uniform refractive index distribution is moremarked in the case of a resin in which fine particles are dispersed. Thereason for this is that as the uncured resin flows during curing, fineparticles also flow, which causes a difference in the content of fineparticles. As a result, the content of fine particles is low in aportion which cures quickly, and the content of fine particles is highin a portion which cures slowly. In particular, in the case of afluororesin, it has been confirmed that, since the viscosity is lowerthan other photo-curable resins, the flowability is higher, and thenon-uniform refractive index distribution is still more marked.

FIG. 1A is a cross-sectional view of a commonly-used diffraction grating100. Inclined surfaces 111 which are optically effective surfaces andwall surfaces 112 which define the height of the grating constitute adiffraction grating shape. In a diffractive optical element, when thediffraction grating 100 is composed of a photo-curable resin material inwhich fine particles of a metal oxide are dispersed, as described above,due to the difference in the curing rate, a non-uniform refractive indexdistribution occurs in the cured diffraction grating.

FIGS. 1B to 1D are each a schematic view showing the refractive indexdistribution state, in which the light application direction to thediffraction grating is indicated by arrows. FIG. 1B shows a case wherelight is applied substantially perpendicularly to inclined surfaces 111of a diffraction grating 101. In this case, in the refractive indexdistribution, the refractive index gradually varies in a directionperpendicular to the inclined surfaces 111. Furthermore, FIG. 1C shows acase where light is applied to each inclined surface 111 of adiffraction grating 102 in a direction deviated from the directionperpendicular to the inclined surface 111 toward the wall surface 112.In this case, in the refractive index distribution, the refractive indexgradually varies from the apex formed by the inclined surface 111 andthe wall surface 112 in the inclined surface 111 direction and in adirection perpendicular to the wall surface 112. That is, a refractiveindex distribution is formed such that the refractive index varies fromleft to right in each groove of the diffraction grating in FIG. 1C.Furthermore, FIG. 1D shows a case where light is applied to eachinclined surface 111 of a diffraction grating 103 in a directiondeviated from the direction perpendicular to the inclined surface 111toward the opposite side of the wall surface 12. In this case, in therefractive index distribution, the refractive index gradually variesfrom right to left in each groove of the diffraction grating in FIG. 1D.

As curing proceeds, the flowability of the photo-curable resin materialdecreases. Therefore, the variation in the refractive index decreases asthe distance from the light source increases. In reality, whenirradiated light is ultraviolet light or the like, the light isrefracted due to the variation in the internal refractive index of thediffraction grating 103. Here, in order to facilitate the description,it is assumed that such refraction does not occur.

FIGS. 2A to 2C each show light paths when one of the diffractiongratings 101 to 103 shown in FIGS. 1B to 1D is incorporated into anoptical system including a plurality of lenses.

FIG. 2A shows light paths when the diffraction grating shown in FIG. 1Bis used. As is evident from FIG. 2A, parallel incident light rays 101 aand 101 b pass through regions having the same refractive indexdistribution. Consequently, the incident light rays 101 a and 101 bwhich have passed through the diffraction grating are refractedsubstantially in the same manner and emitted while maintaining theparallel state. Consequently, by setting optical design values inconsideration of the refracted value in advance, it is possible torealize high diffraction efficiency.

FIG. 2B shows light paths when the diffraction grating shown in FIG. 1Cis used. As is evident from FIG. 2B, parallel incident light rays 102 aand 102 b pass through regions having completely different refractiveindex distributions. In particular, due to the difference in refractiveindex from the wall surface 112, the incident light rays 102 a and 102 bwhich have passed through the diffraction grating are emitted incompletely different directions. In reality, it is very difficult todetermine how the incident light ray 102 a is refracted, and the opticaldesign values cannot be corrected, resulting in a decrease indiffraction efficiency.

FIG. 2C shows light paths when the diffraction grating shown in FIG. 1Dis used. As is evident from FIG. 2C, parallel incident light rays 103 aand 103 b pass through regions having different refractive indexdistributions. However, the difference in the refractive indexdistribution is slight, and the incident light rays 103 a and 103 b arerefracted substantially in the same manner and emitted while maintainingthe parallel state. Consequently, as in FIG. 2A, by setting opticaldesign values in consideration of the refracted values in advance, it ispossible to realize high diffraction efficiency.

On the basis of what has been described above, as the diffractiongrating actually formed, the diffraction grating 101 shown in FIGS. 1Band 2A may mostly be chosen. With respect to the diffraction grating 103shown in FIGS. 1D and 2C, although being inferior to the diffractiongrating 101, the refractive index distribution can be limited to a rangethat can be sufficiently handled through optical design. However, in thecase of the diffraction grating 102 shown in FIGS. 1C and 2B, it may bedifficult to determine the path of the incident light ray 102 a, anddiffraction efficiency is decreased. Consequently, in one embodiment,the incident angle of ultraviolet light to be applied is 90 degrees withrespect to the inclined surfaces 111, and the incident angle should bedeviated from the direction perpendicular to each inclined surface 111toward the thinner portion of the resin layer in the inclined surface.

FIG. 3 is a cross-sectional view showing a laminated diffractive opticalelement 10 according to a first embodiment. Reference numeral 1represents a meniscus-shaped first glass lens substrate, and referencenumeral 2 represents a convex second glass lens substrate. A first resinlayer 3 and a second resin layer 4 are disposed, in that order from thefirst glass lens substrate 1 side, between the first and second glasslens substrates 1 and 2. The first resin layer 3 and the second resinlayer 4 are each composed of a photo-curable resin which is cured bylight, such as ultraviolet light. Fine metal particles are dispersed inthe second resin layer 4. The boundary surface between the first resinlayer 3 and the second resin layer 4 has a diffraction grating shapewith a saw-toothed cross-section. Because of the boundary surface havingthe diffraction grating shape, the laminated diffractive optical element10 exhibits a diffraction effect. The boundary surface having thediffraction grating shape includes inclined surfaces 11 which areoptically effective surfaces and wall surfaces 12 which define theheight of the grating.

In the laminated diffractive optical element 10 shown in FIG. 3,inclination angles of the inclined surfaces 11 are not constant. Theinclination angles gradually vary from the center toward the peripheryof the laminated diffractive optical element 10. In ordinary opticaldesign, when the inclined surfaces 11 are connected to each other, anaspherical shape is formed. Therefore, the inclination angles of theinclined surfaces 11 become gentler with increasing distance from thecenter toward the periphery of the laminated diffractive optical element10.

Next, a method for manufacturing a laminated diffractive optical element10 will be described with reference to FIGS. 4A and 5D. First, as shownin FIG. 4A, an appropriate amount of a photo-curable resin material 3 afor forming a first resin layer 3 is added dropwise onto a molding dieobtained by machining a plating layer of NiP or the like. Thephoto-curable resin material 3 a is incorporated with a reactioninitiator so that photo-curing can be initiated. Next, as shown in FIG.4B, a first glass lens substrate 1 is arranged so as to cover thephoto-curable resin material 3 a. Next, as shown in FIG. 4C, bygradually lowering the glass lens substrate 1, the dropwise addedphoto-curable resin material 3 a and the glass lens substrate 1 arebrought into contact with each other, and a space between the glass lenssubstrate 1 and the molding die 5 is filled with the photo-curable resinmaterial 3 a such that bubbles are not included therein. Incidentally,in order to improve adhesion between the glass lens substrate 1 and thephoto-curable resin material 3 a in advance, a silane coupling agent isapplied by a spinner onto the surface of the glass lens substrate 1,followed by drying with an oven.

Next, as shown in FIG. 4D, the photo-curable resin material 3 a is curedand integrated with the first glass lens substrate 1 by applyingultraviolet light through the first glass lens substrate 1. Next, asshown in FIG. 4E, the first glass lens substrate 1 and the first resinlayer 3, which have been integrated together, are released from themolding die 5 by lifting the peripheral portion of the first glass lenssubstrate 1. Thereby, a diffractive optical element including the firstglass lens substrate 1 and the resin layer 3 is obtained.

In the case of a laminated diffractive optical element including aplurality of resin layers, the following steps are further carried outafter the steps shown in FIG. 4A to 4E. As shown in FIG. 5A, a resinmaterial 4 a for forming a second resin layer 4 is added dropwise onto asecond glass lens substrate 2. Next, as shown in FIG. 5B, the firstglass lens substrate 1 and the first resin layer 3, which have beenintegrated together and formed in the step shown in FIG. 4E, arearranged, with the first resin layer 3 being directed downward, over thephoto-curable resin material 4 a. The photo-curable resin material 4 ais prepared by dispersing fine metal oxide particles in a fluororesinmaterial, and is incorporated with a reaction initiator so thatphoto-curing can be initiated. Next, as shown in FIG. 5C, by graduallylowering the first glass lens substrate 1, the dropwise addedphoto-curable resin material 4 a and the first resin layer 3 are broughtinto contact with each other, and a space between the second glass lenssubstrate 2 and the first resin layer 3 is filled with the photo-curableresin material 4 a such that bubbles are not included therein.

Next, as shown in FIG. 5D, by applying ultraviolet light through thefirst glass lens substrate 1, the photo-curable resin material 4 a iscured, and the first glass lens substrate 1, the first resin layer 3,the second resin layer 4, and the second glass lens substrate 2 areintegrated together. Then, a sealing material is applied to theperipheral portions of the glass lens substrates to seal the inside.Thereby, a laminated diffractive optical element 10 shown in FIG. 3 isobtained.

As described above, the inclination angles of the inclined surfaces 11become gentler with increasing distance from the center toward theperiphery of the laminated diffractive optical element 10. Consequently,if ultraviolet light is applied in a given direction in the steps shownin FIGS. 4D and 5D, there is an increased possibility that, in any ofthe inclined surfaces 11, the refractive index distribution as the oneshown in FIGS. 1C and 2B may occur.

FIG. 6A is a cross-sectional view showing in detail the step of applyingultraviolet light to the photo-curable resin material 4 a shown in FIG.5D, and FIG. 6B is a cross-sectional view showing in detail thesubstantial part thereof. A lens unit 6 is arranged above the secondglass lens substrate 2. Ultraviolet light applied is refracted by thelens unit 6 and enters the diffraction grating in a directionsubstantially perpendicular to the inclined surfaces 11 over the entireregion of the diffraction grating shape. In FIG. 6B, reference numerals20 a and 20 b represent ultraviolet light paths, which enter differentinclined surfaces 11 in the substantially perpendicular direction. Thelens unit 6 is designed in consideration of the refractive indexdistributions described with reference to FIGS. 1A to 1D. In thelaminated diffractive optical element 10 according to this embodiment,the inclination angles of the inclined surfaces 11 gradually vary fromthe center toward the periphery of the laminated diffractive opticalelement 10. Therefore, lenses used in the lens unit 6 contain asphericalcomponents.

An ultraviolet light lamp has an emission spectrum over a widewavelength band from ultraviolet to visible regions. Since thewavelength region that causes a curing reaction of the photo-curableresin material 4 a is wide at 250 to 400 nm, the wavelength region oflight used for curing is limited to 360 to 370 nm by a UV-bandpassfilter 7. The reason for this is that since the lens unit 6 includesrefractive lenses, the refractive index of the glass material of thelens unit 6 varies depending on the wavelength of light incident on thelens unit 6, and light rays of a plurality of wavelengths are applied atdifferent incident angles.

The lens unit may be designed such that ultraviolet light is appliedsubstantially perpendicularly to all the inclined surfaces 11, asdescribed above. In such a case, the state shown in FIGS. 1B and 2A isbrought about. However, depending on the shape of the diffractiongrating, there may be a case where it is difficult to apply ultravioletlight perpendicularly to all the inclined surfaces 11 in terms ofoptical design. In such a case, with respect to all the inclinedsurfaces 11, the incident angle of ultraviolet light to be appliedshould be at least deviated from the apex formed by the inclined surface11 and the wall surface 12 toward the inclined surface 11 side. That is,it is important that all the inclined surfaces 11 are not in the stateshown in FIGS. 1C and 2B.

Furthermore, in the step of applying ultraviolet light to thephoto-curable resin material 3 a shown in FIG. 4D, as in the case shownin FIGS. 6A and 6B, a lens unit is used so that ultraviolet light isapplied perpendicularly to the inclined surfaces 11. However, thephoto-curable resin material 3 a does not have fine metal particlesdispersed therein, and therefore is not greatly affected by therefractive index distribution, and it may not be necessary to add a lensunit.

In the first embodiment, ultraviolet light is applied to thephoto-curable resin material 3 a through the first glass lens substrate1, and ultraviolet light is applied to the photo-curable resin material4 a through the first glass lens substrate 1. In a second embodiment, asshown in FIG. 7A, by using a material that transmits ultraviolet lightfor a molding die 8, it is possible to cure the photo-curable resinmaterial 4 a from the molding die 8 side. FIG. 7B is a cross-sectionalview showing the substantial part of FIG. 7A. In FIGS. 7A and 7B,reference numerals 30 a and 30 b represent ultraviolet light paths,which enter different inclined surfaces 11 in a direction perpendicularto the inclined surfaces. As for the molding die 8, it is possible touse a master composed of a UV-curable resin containing methacrylate as amajor component in which a NiP plating layer is machined into a gratingshape in advance. In this case, a lens unit 9 is designed such thatultraviolet light enter all the inclined surfaces 11 of the resinmaterial 4 in the perpendicular direction.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all modifications and equivalent structures and functions.

1. An apparatus comprising: a glass lens substrate; and a resin layercomposed of a photo-curable resin disposed on the glass lens substrate,the resin layer having a diffraction grating shape including a pluralityof inclined surfaces and wall surfaces, wherein a direction ofrefractive-index variation of the resin layer is substantiallyperpendicular to the inclined surfaces or deviated from theperpendicular direction toward a thinner portion of the resin layer withrespect to each inclined surface.
 2. The apparatus according to claim 1,wherein fine metal particles are dispersed in the resin layer.
 3. Anapparatus comprising: a first glass lens substrate; a second glass lenssubstrate; and a first resin layer and a second resin layer disposed andlaminated between the first glass lens substrate and the second glasslens substrate, the first resin layer and the second resin layer eachbeing composed of a photo-curable resin, a boundary surface between thefirst resin layer and the second resin layer having a diffractiongrating shape including a plurality of inclined surfaces and wallsurfaces, wherein a direction of refractive-index variation of thesecond resin layer is substantially perpendicular to the inclinedsurfaces or deviated from the perpendicular direction toward a thinnerportion of the resin layer with respect to each inclined surface.
 4. Theapparatus according to claim 3, wherein fine metal particles aredispersed in the second resin layer.
 5. A method for manufacturing anapparatus including a glass lens substrate and a resin layer disposed onthe glass lens substrate, the resin layer having a diffraction gratingshape, the method comprising: supplying a photo-curable resin materialto a molding die having the diffraction grating shape including aplurality of inclined surfaces and wall surfaces; bringing the glasslens substrate into contact with the photo-curable resin materialsupplied to the molding die to fill a space between the molding die andthe glass lens substrate with the photo-curable resin material; curingthe photo-curable resin material by applying light in a directionsubstantially perpendicular to the inclined surfaces of the diffractiongrating shape of the molding die or in a direction deviated from theperpendicular direction toward a thinner portion of the resin layer withrespect to each inclined surface to integrate the glass lens substratewith the resin layer having the diffraction grating shape; and releasingthe glass lens substrate and the resin layer, which have been integratedtogether, from the molding die.
 6. The method according to claim 5,wherein the photo-curable resin material is a UV-curable resin, thelight to be applied is ultraviolet light, the molding die is composed ofa material that transmits ultraviolet light, and the ultraviolet lightis applied through the molding die.
 7. The method according to claim 5,wherein the light application direction is determined by a lens unitdisposed between a light source and the glass lens substrate and theresin layer.
 8. A method for manufacturing an apparatus including afirst glass lens substrate, a second glass lens substrate, and a firstresin layer and a second resin layer disposed and laminated between thefirst glass lens substrate and the second glass lens substrate, themethod comprising: supplying a first photo-curable resin material to amolding die having a diffraction grating shape including a plurality ofinclined surfaces and wall surfaces; bringing the first glass lenssubstrate into contact with the first photo-curable resin materialsupplied to the molding die to fill a space between the molding die andthe first glass lens substrate with the first photo-curable resinmaterial; curing the first photo-curable resin material by applyinglight to integrate the first glass lens substrate with the first resinlayer; and releasing the first glass lens substrate and the first resinlayer, which have been integrated together, from the molding die;supplying a second photo-curable resin material to the second glass lenssubstrate; bringing the first glass lens substrate and the first resinlayer, which have been integrated together, into contact with the secondphoto-curable resin material supplied to the second glass lens substrateto fill a space between the first resin layer and the second glasssubstrate with the second photo-curable resin material; and curing thesecond photo-curable resin material by applying light in a directionsubstantially perpendicular to the inclined surfaces or in a directiondeviated from the perpendicular direction toward a thinner portion ofthe resin layer with respect to each inclined surface to integrate thefirst glass lens substrate and the first resin layer with the secondglass lens substrate and the second resin layer.
 9. The method accordingto claim 8, wherein the second resin layer is composed of a fluororesinin which fine metal particles are dispersed.
 10. The apparatus accordingto claim 2, wherein the resin layer is composed of a fluororesin inwhich fine metal particles are dispersed.
 11. The method according toclaim 5, wherein the resin layer is composed of a fluororesin in whichfine metal particles are dispersed.
 12. The method according to claim 8,wherein the first or the second photo-curable resin material is aUV-curable resin, the light to be applied is ultraviolet light, and themolding die is composed of a material that transmits ultraviolet light.13. The method according to claim 8, wherein the light applicationdirection is determined by a lens unit disposed between a light sourceand, the first glass lens substrate and the first resin layer.
 14. Themethod according to claim 8, wherein the light application direction isdetermined by a lens unit disposed between a light source and the secondglass lens substrate.